1. Field of the Invention
The present invention relates generally to infusion pump systems for the delivery of infusion formulations and, in particular, to an implantable infusion pump system and process for delivering insulin to a user based in part on user-initiated signals which correspond to events which may affect the glucose level of the user.
2. Description of Related Art
In the medical arts, implantable infusion pumps are used for the programmed delivery of measured doses of an infusion formulation. (An infusion formulation is defined in the present disclosure as the substance being delivered by the infusion pump. This substance may comprise either a mixture of different components or it may be a single, pure substance.) A typical example of such use is the intraperitoneal delivery of an insulin formulation. FIG. 1 illustrates an example of this use. As shown in FIG. 1, an implantable infusion pump (IIP) 10 may be implanted in a patient below the skin and above the muscle in the abdomen. The IIP 10 will then dispense an infusion formulation (such as an insulin formulation) through the peritoneum wall 12 via a catheter 14. A lead 16 may connect IIP 10 to a sensing device (not shown) that is used to regulate the delivery of the infusion formulation.
In the case where the infusion formulation is an insulin formulation, the sensing device may regulate the delivery of the insulin formulation by sensing the levels of glucose in the patient. The delivery of the insulin formulation may then be performed in two ways. Information about the sensed glucose level may be provided to the patient (or to the patient's physician) through a communication device associated with the pump. The patient (or physician) would then manually administer an appropriate amount of the insulin formulation in accordance with the sensed glucose level. Alternatively, the sensed glucose level may be provided to a control device associated with the pump (such as a processor or other computing element) for controlling activation of the pump to deliver an appropriate amount of the insulin formulation in accordance with the sensed glucose level.
As discussed above, a typical use for an implantable infusion pump is the intraperitoneal delivery of an insulin formulation. Insulin is a protein hormone normally formed within the human pancreas. Because it regulates carbohydrate (sugar) metabolism, insulin is required for normal metabolic function. More specifically, insulin helps the body process glucose. To avoid medical problems, glucose levels should be maintained within a specific range. A normal range for glucose in the human body may be between 85 and 120 mg/dl.
In a non-diabetic person, insulin is secreted by the pancreas in small amounts throughout the day (basal rate of insulin secretion). In addition, the amount of insulin secreted by the pancreas may be modified under certain circumstances. For example, the pancreas of a non-diabetic person normally secretes larger amounts of insulin (bolus rate of insulin secretion) when the person ingests a meal to prevent postprandial hyperglycemia, i.e., abnormally increased sugar content in the blood.
In contrast to the non-diabetic person, a diabetic person's pancreas may not secrete the required amount of insulin. Thus, the diabetic person has to somehow artificially introduce the insulin into the body. One method of introducing the insulin is by the conventional insulin formulation injection method using a syringe. Using this method, the body's glucose level may be monitored (for example, by checking a blood sample) and the amount of insulin to be injected may be adjusted accordingly. For example, after a meal the glucose level may be monitored and an appropriate amount of insulin may be injected into the bloodstream of the individual.
In the alternative, a diabetic person may choose to use an infusion pump such as the implantable infusion pump described above and shown in FIG. 1. By using an infusion pump, a diabetic person may be able to adjust insulin delivery rates for the pump in accordance with the user's needs. These needs may be determined based on prior experience and/or the results of glucose monitoring (for example, by a sensing device in combination with a communication device). As an example, the basal and bolus delivery rates of an infusion pump may be adjusted in this manner.
In addition, infusion pumps may be engineered to function as an artificial pancreas. Such an infusion pump may deliver a specific amount of insulin formulation at specific intervals. As discussed above, a sensing device associated with the pump may monitor the glucose level of the user and the glucose level may then be used by the pump to automatically regulate the delivery of the insulin formulation. The automatic regulation may be carried out by a processor or other computing element associated with the pump.
The processor or other computing element may execute a closed-loop algorithm which may adjust insulin formulation delivery as a function of, for example, the rate of change over time of a sensed glucose level. These processes may be transparent to the user. Thus, the infusion pump in combination with a sensing device and closed-loop algorithm may be very beneficial to a diabetic person by automating the tasks of monitoring glucose levels and introducing an appropriate amount of insulin formulation based on the glucose level, with minimal input from the user (or the user's physician).
However, a problem exists with the method described above for the automated delivery of insulin using an infusion pump. The problem results from the fact that an individual's glucose level may be significantly affected by certain daily events. For example, when a person ingests food, glucose levels may rise due to ingested carbohydrates (sugars). In addition, it is believed that sleep affects glucose levels due to changes in the rate of glucose metabolism when a person sleeps. An individual's stress level may also affect glucose metabolism by increasing glucose levels in the bloodstream. Furthermore, the ingestion of medications may affect glucose levels within the body.
A properly functioning sensing device may detect a change in glucose level due to any of the events described above and provide the change in glucose level as an input to the closed-loop algorithm which may, in turn, provide an output to the pump to properly adjust the delivery of insulin formulation accordingly. However, in the case of an erroneous input to the closed-loop algorithm, for example, as a result of a malfunctioning sensing device, an erroneous glucose level may be indicated, leading to an erroneous adjustment in the amount of insulin delivered to the pump user. Under certain circumstances, such an error may result in extreme harm including death) to the pump user.
Furthermore, it is believed that the body of a person merely anticipating the ingestion of a meal may have an increased level of insulin secretion. This increased insulin secretion may occur before any increase in glucose level can be detected by a sensing device. It is further believed that one reason for this leading insulin secretion reflex may be that the body is compensating, by early release of the insulin, for the time required for the insulin to react with the glucose. The secretion of insulin associated with meal anticipation is believed to lead any significant rise in glucose level by as much as 15-20 minutes. With present infusion pump systems for delivery of insulin formulation, such leading insulin secretion reflex may not be replicated, because the delivery of insulin by the pump may not occur until triggered by the detection of glucose by the sensing device.
Accordingly, there is a demand for an infusion pump system and process for delivery of insulin formulation which provides safety limits that may be used in conjunction with a closed-loop algorithm for adjusting insulin formulation delivery. The safety limits verify that levels of glucose detected by the infusion pump system's sensing device are consistent with events that may significantly affect the glucose level. In addition, there is a need for an infusion pump system and process for delivery of insulin formulation which may more accurately replicate the body's leading insulin secretion reflex.